We will now proceed to consider the different parts of a kingpost roof in detail.
The Wall Plates are pieces of timber imbedded in mortar on the tops of the walls to carry the ends of the tie beam and distribute its weight. They are sometimes bolted down to the wall so as to secure the roof in case of wind getting under it and tending to lift it.
The tie beam is sometimes notched or cogged upon them, but is generally only nailed to them.
It is an advantage to have the wall plate over the centre of the wall, so as to bring the weight fairly upon the masonry, but this increases the bearing of the tie beam and causes expense. Wall plates are therefore generally placed so as to be flush with the inner faces of the walls.
At the angles of buildings the wall plates are halved, dovetailed, or notched into one another, and well spiked together, and halved or scarfed wherever it is necessary to join them in the direction of their length: they should, however, be in long pieces, so as to avoid this as much as possible.
In roofs of very wide span two wall plates parallel to one another, and a few inches apart, are sometimes placed on each wall, so as to secure the tie beam more firmly.
Templates are long fiat stones frequently substituted with great advantage for wall plates. They are frequently made of wood, but stone has the great advantage of not being subject to decay or destruction by fire.
The tie beams either merely rest upon them or are secured by joggles.
Wall plates which are not continuous, but which are placed under the ends of the trusses in pieces only sufficiently long to distribute the weight, are also sometimes called templates.
The Tie Beam.1 - As far as the roof itself is concerned, this member has nothing to do but to hold in the feet of the rafters to prevent them from spreading, and it would thus be subject only to a tensile stress.
1 Sc. sometimes called Tie joist.
In many cases, however, it carries the ceiling joists (see Figs. 323 to 326), and it has then to bear the cross strain caused by the weight of the ceiling.
To prevent it from sagging or drooping in the centre, the tie beam should be supported at one or more points in its length. As a rule, there should not be more than 12 or 14 feet between the points of support.
In a king-post roof there is only one such point of support, and it is in the centre of the tie beam (see p. 173).
The tie beam receives the feet of the rafters in oblique mortises (see p. 70), the joints being further secured by straps or bolts.
As a point of construction, it is better that the joint between the foot of the rafter and the tie beam should be over the wall, as shown in Figs. 321, 325, instead of within it, as in Figs. 322, 326; but as the latter position allows a wider span between the walls, with the same amount of timber in the roof, it is very frequently adopted.
In such a case, iron, stone, or wood corbels (see Figs. 202, 321) or brackets (Fig. 338) are often provided, so that the bearing of the tie beam is reduced, and support is afforded to it just below the points where the rafters bear upon it.
The ends of the tie beam are notched or cogged, and nailed upon the wall plates, and should be left with a free circulation of air around them. The tie beam is frequently "cambered" in the middle to allow for sagging, so that after it has taken its bearing it may be horizontal. When there are ceiling joists attached to the tie beam, the same object may be effected by keeping them a little higher at the centre (see p. 141, Floors), varying the depth of the notches on their upper sides.
The centre of the tie beam is upheld by being strapped (see Fig. 323) to the king post. The shrinking of the timbers will cause the tie beam to separate from the foot of the king post; the strap should therefore be furnished with adjusting wedges (c c, Fig. 324), which can be tightened up to counteract this.
A slot or rectangular hole is made in the strap and through the post, to receive these iron wedges, or, as they are technically called, "cotters;"1 they are enabled to slide easily, and prevented from crushing into or indenting the wood by iron shields above and below called "gibs" (g g, Fig. 324), which are so formed as to clip the sides of the strap and keep them close to the king post; the manifest effect of driving the wedges inwards is to raise the upper gib and strap, and with it the tie beam it supports.
1 Sometimes called keys.
Fig. 324 shows the position of the wedges before tightening up.
The slots should be so arranged that there is, before driving the wedges, a space in the king post at x above the upper gib, and one in the strap at y below the lower gib, so as to admit of the strap being raised until the tie beam is as close up to the king post as possible.
The King Post, or "King Piece" is intended merely as a tie to hold up the centre of the tie beam and prevent it from sagging, for which purpose it is united to it by the strap or stirrup just described.
The head (see Fig. 323) is if possible enlarged, so as to afford a bearing perpendicular to the pressure of the principal rafters (see p. 72), and bevelled and mortised to receive their upper ends, which are tenoned into it.
The top of the head may be left square, as in Fig. 183, when there is no common rafter immediately over the Principal, or bevelled off parallel to the backs of the common rafters where they occur (Fig. 322). A notch is cut in it to receive the ridge (r).
The foot (Fig. 323) is similarly arranged to receive the feet of the struts.
The lower the block at the foot of the king post the better will the struts be placed for taking the strain upon them.